In Sweden wastewater treatment plants annually collect more than 200,000 tons of sludge dry matter (DM), equivalent to 1 million tons of dewatered sludge. A wastewater treatment plant with anaerobic digestion produces 20 kg of DM per person per year. This amounts to 0.65 kg of phosphorus per person per year.

The total economic value of recovered phosphorus and nitrogen is estimated at about SEK 230 million at a price of SEK 20 / kg for phosphorous and SEK 10 / kg for nitrogen.

Approximately 80 percent of the 150,000 tons of dewatered sewage sludge Ragn-Sells handles each year is used on farmland.
Source: Ragn-Sells

The ‘circular economy’ describes how to recycle, repair and fix what many consider ‘junk’ – which for others is a reusable asset. The ‘linear economy,’ its counterpart, is based on the extraction of raw materials, production in some form, their use and subsequent discarding in landfills. This latter model works as long as it is assumed that the earth’s resources are infinite, readily available and cheap.

It has become abundantly clear in recent decades that these resources are neither infinite nor cheap, and that raw material extraction poses negative consequences for people and the environment. Resources can certainly be used more efficiently, but in the long term that is not enough. And to stop consuming altogether is not a viable option for businesses or consumers.

A business case for the ‘Circular economy’

The phosphorus cycle – Innovative sludge incineration technology

The circular economy is a model that allows both consumption and growth – in a smart and sustainable way. It basically ties the two ends of the linear economy into a circle in such a way that:

-Used biological materials are composted and thus renewed
-Materials like plastics and alloys are designed for reuse
-The use of toxic chemicals is reduced as much as possible
-Renewable energy sources are used when possible

Many companies understand these ideas and have created entirely new business models. One example is recycling company Ragn-Sells that together with Easy Mining Sweden has developed a way to recycle phosphorus.

‘Peak phosphorus’ threatens food security

Access to phosphorus is a precondition for global food production. Almost 90 percent of extracted phosphorus is used in fertilizers, animal feed and plant protection products such as herbicides. But there is a global shortage, a situation known as ‘peak phosphorus,’ which like ‘peak oil,’ stipulates that global maximum production will soon be reached. According to some estimates, ‘peak phosphorus’ could occur within 80 years, and total recoverable reserves will run out in about 350 years.

The main problem is that the phosphorus compounds that plants and animals absorb are not properly redistributed in their lifecycle. Phosphorus ends up instead as sludge in wastewater treatment plants. Europe burns 30 percent of its sewage sludge and about 40 percent of all its slaughterhouse waste. Approximately 130,000 tons of phosphorus ash is largely landfilled. Incinerating sewage sludge is difficult and costly because it contains 70 percent water. This means sludge needs to be pre-dried before combustion, which is both time consuming and expensive.

Sustainable phosphorus extraction

“Sewage sludge contains significant amounts of high quality phosphorus and Ragn-Sells handles a big chunk of this. Much of it is approved for agricultural use and is returned to arable land, and some is not. In the case of sludge that contains high concentrations of heavy metals, the only option is combustion in which case the phosphorus goes out of circulation and ends up in landfills,” says Anders Kihl, Business Area Manager Construction in Ragn-Sells AB.

“We want to bridge the gap between country and city. The new method is based on sewage sludge incinerated with recycled wood chips where we extract different phosphorus compounds from the ashes. Dr. Yariv Cohen at the Swedish University of Agricultural Sciences developed a method to separate phosphorus and other substances from ash, sewage and mining waste. The basic principle is that the ash gets dissolved in sulfuric acid and by chemical methods to precipitate iron, aluminum, heavy metals, gypsum and phosphorus from the solution. Phosphorus can thus be extracted in the form of ammonium phosphate and calcium phosphate,” says Kihl.

“Together with Easy Mining Sweden, we have demonstrated the possibility of burning sludge with wood chips to recover phosphorus. A 60 percent wood chip concentration yields moisture content of up to 50 percent, about the same as a biofuel. And the aerobic decomposition process is almost odorless making sludge and chip handling a non-issue for any neighbors,” says Kihl.

“Combustion destroys all organic contaminants and drug residues. Heavy metals are collected and deposited in our modern and safe landfills. And through combustion, energy is recovered in the sludge and the process water gets re-circulated resulting in a sandy pool.”

Plans for large-scale recovery

“With this project, we have shown the technical possibility of extracting phosphorus from low quality sludge in a cost effective manner. We hope that future regulations will reflect this. Without clear national guidelines, the risk exists that the sludge gets incinerated without consideration of phosphorus’ benefit to agriculture. We are now planning a large-scale production facility to process approximately 30,000 tons per year. For it to be profitable, we need to get a phosphorus to ash ratio of five percent, which is where we are now,” concludes Kihl.